The present invention relates to a carrier-to-noise detector for digital transmission systems.
The operating performance of the demodulator of a digital transmission system is evaluated by the ratio of per-bit energy to noise (E.sub.b /N.sub.o) of a demodulated digital signal which is defined as: ##EQU1## where C/N is a carrier to noise ratio, B, the equivalent noise bandwidth of the demodulator and R, the data transmission rate which is equal to the symbol rate in a 2-PSK system and equal to twice the symbol rate in a 4-PSK system. The C/N ratio is therefore a determining factor for system evaluation. In satellite communications systems, for example, the evaluation of a satellite channel is made by inserting a band-pass filter having a passband narrower than the bandwidth of the satellite transponder at the input of a demodulator. A test carrier having a frequency corresponding to the center frequency of the band-pass filter is transmitted to measure the level of power at the output of the filter which represents the total value (C+N). The carrier is then removed and the power level is again measured as a representation of the noise component N. The carrier component C is then obtained by subtracting the noise N from the total value (C+N) and finally the value C is divided by the noise value N to obtain the ratio C/N. The equivalent noise bandwidth of the band-pass filter corresponds to the constant B.
The procedures are carried out manually and the transmission system must, therefore, be interrupted.